Exploration and production of deeper, lower quality oil and gas reserves has challenged refiners and gas processors with feedstocks having significantly higher sulfur content, increasing corrosion risks and other undesired impurities. Amine units for amine gas treating (also known as “gas sweetening” or “acid gas removal”) refers to a group of processes or units that use aqueous solutions of various alkanolamines (commonly referred to simply as amines) for the removal of H2S and CO2 from gases. The removal is driven by either required product specification, e.g., natural gas that contains less than 4 ppm H2S, or by environmental permission requirements, and/or by the minimization of corrosion problem of process equipment.
Amine treating units are commonly used in refineries as well as in petrochemical plants, natural gas processing plants, and other industries, for the removal of H2S and CO2 from gases. When volatile mercury is present in the gases to the amine units, it will appear in the products from the units, e.g., the sweetened gas (after removal of H2S and CO2) and rich amine containing H2S and CO2. Mercury, CO2 and H2S are to be removed from the sweetened gas. Mercury in these products can impact the operations downstream from the amine units.
Glycol dehydrators are sometimes used in to dry natural gas, e.g., the sweetened gas from the amine unit, to specifications. The glycol absorbs the moisture forming a dry gas. Wet glycol is recycled by vaporizing water at low pressure and at elevated temperatures. Methane and other hydrocarbons are somewhat soluble in triethylene glycol, particularly at high pressure and ambient temperature. These hydrocarbons are vaporized from the glycol as the pressure is reduced and temperature is increased. If mercury is present in the natural gas, a portion can appear in either the dry gas or vent to the atmosphere from glycol dehydrators.
Absorbers have been used in the prior art to remove heavy metals such as mercury from the sweetened gas to specifications, e.g., 0.1 μg/m3, with the use of an adsorbent bed of carbon, zeolite, or supported metals known in the art. The adsorbents are rather expensive and require disposal as a hazardous waste, or shipped to a re-processing facility.
There is still a need for improved methods and systems to remove volatile mercury from fluids such as natural gas, vent gas or flue gas, allowing for the optimization of plant operations to minimize the equipment corrosion, mercury emission and any HES risks associated with mercury.